Typical one-metre speech at 60-70 dB is optimally tuned for auditory communication

Evidence

A good reference in the middle frequency range is to remember that typical speech measured from a distance of one metre is about 60–70 dB. This is the optimal level for communication, since it is not too loud to be harmful for the functioning of hearing but is loud enough to produce a good signal-to-noise ratio in most environments, and it also provides good conditions for temporal and spectral analysis to the auditory system. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 980